Humans and other vertebrates sense sounds over a wide range of frequencies, and being able to hear across this range is critical for properly perceiving the surrounding environment as well as understanding speech. Sound perception originates in the cochlea, which is topologically tuned along its longitudinal axis, with the dista (apical) end tuned to low frequency sounds and the proximal (basal) end tuned to high frequencies. Patterning of the cochlea along this axis of sensitivity is required for proper auditoy function, and the broad, long-term goal of this proposal is to gain an understanding of how this patterning occurs during development. The cochlea contains specialized sensory receptor cells called hair cells (HC), which convert sound into electrical signals, which are then transduced to higher processing centers in the brain. Humans and other mammals incur permanent hearing loss when these auditory HCs are lost. Therapies that replace lost sensory cells must ensure that the new hair cells establish the anatomical patterning essential for generating the tonotopic gradient needed for frequency discrimination. HCs contain apically protruding F-actin rich structures called stereocilia, which form tightly packed arrays of rows of increasing height. There exist structural gradients of stereocilia height and number in the HCs across the tonotopic axis of the chick cochlea. HCs contain ~50 stereocilia that reach a maximum height of 5.5 ?m in the extreme apical (distal) end and ~250 stereocilia that reach 1.5 ?m at the basal (proximal) end (Tilney and Saunders, 1983). These structural differences also have functional importance in the tuning of HCs to different sound frequencies - shorter bundles respond to high frequency sounds and taller bundles respond to low frequency sounds (Frishkopf and DeRosier, 1983; Holton and Hudspeth, 1983). Furthermore, this spatial gradient has also been observed in mammals (Lim, 1980, 1986). This gradient of hundreds of cell phenotypes serves as an excellent readout of the patterning of the cochlea along the tonotopic axis due to its unique quantifiable nature. I have established three aims to gain an understanding of the tonotopic patterning mechanism. I aim to discover whether diffusible signals required for patterning the tonotopic axis of the cochlea are located intrinsically within the avian cochlea. I also aim to sequence the transcriptome across the longitudinal axis of the cochlea to discover differentially expressed genes and signaling pathways that may underlie this patterning. Finally, I aim to modulate candidate signaling pathways in vitro and in ovo to determine whether such manipulations disrupt normal patterning of HC phenotypes along the longitudinal axis of the cochlea.